This application relates to and incorporates herein by reference Japanese Patent Application No. 11-225418 filed on Aug. 9,1999.
The present invention relates to an apparatus for driving an electromagnetic device, which may be used for supplying fuel to an engine of a vehicle.
A conventional electromagnetic device driving apparatus of this type is disclosed in USP 4,605,983 (JPB2-4-42805).
When this apparatus is applied to a high-pressure fuel pump, a conduction current flowing through a coil employed in an electromagnetic valve is driven to reach a predetermined magnitude Ip before a predetermined period of time lapses after the start of the valve driving so that the movable body of the valve may be actuated quickly. Thereafter, the conduction current is reduced to a small holding magnitude Ih ( less than Ip) which is large enough only for holding the movable body of the valve at the current position. As a result, a good driving response of the electromagnetic valve or the operating response of the valve body can be attained by merely supplying a reduced conduction current or a reduced driving current.
In the above current control, a switching device such as a MOSFET is connected in series with the coil of the electromagnetic valve and on/off-controlled to maintain the magnitude of the conduction current. When the switching device is turned off, an arc-extinguishing current Iex flows. It is likely that this arc-extinguishing current influences a conduction current, which flows in the coil, when the MOSFET is turned on next time. Thus, the valve driving characteristics is lessened.
The above problem occurs irrespective of the type of the electromagnetic valve, that is, a normally closed-type or a normally open-type. In addition, the above problem is also encountered in the case of an electromagnetic valve employed as a fuel injector (a fuel injection valve) for supplying fuel to the engine.
It is thus an object of the present invention to provide an electromagnetic device driving apparatus capable of ensuring a good driving characteristic.
According to the present invention, a driving apparatus for an electromagnetic device comprises a conduction control circuit. The conduction control circuit turns on and off a switching device so that a conduction current flowing through the electromagnetic device has a magnitude at around a predetermined current limit value while a driving signal is applied to the conduction control circuit. The predetermined current limit value used in the conduction control circuit is set at a first current limit value during a first predetermined period from a start of the driving signal. When the predetermined first period has lapsed, the predetermined current limit value is switched to a second current limit value for a predetermined second period. The second limit value is smaller than the first current limit value.
A first off-time of the switching device for attaining the first current limit value in the first period is set to be shorter than a second off-time of the switching device for attaining the second current limit value in the second period. Alternatively, a first on-time of the switching device for attaining the first current limit value in the first period is set to be shorter than a second on-time of the switching device for attaining the second current limit value in the second period.